使用Java或Groovy为SQLite编写用户定义的SQL函数？

使用SQLite,可以使用C api或PHP轻松添加用户定义的SQL函数.但它是否也可以使用Java或Groovy？

既然问题是关于Java或Groovy的解决方案,有人可以在这里寻找Java解决方案(就像我做的那样)

下面你可以看到验证的简单函数,如果提供日期是在同一天:

public class IsSameDay extends Function {

    @Override
    protected void xFunc() throws SQLException {
        if (args() != 2) {
            throw new SQLException("IsSameDay(date1,date2): Invalid argument count. Requires 2, but found " + args());
        }
        try {
            DateTime t1 = DateTime.parse(value_text(0).replace(" ", "T"));
            DateTime t2 = DateTime.parse(value_text(1).replace(" ", "T"));
            if (t1.getYear() == t2.getYear() && t1.getDayOfYear() == t2.getDayOfYear()) {
                result(1);
            } else {
                result(0);
            }
        } catch (Exception exception) {
            throw new SQLDataException("IsSameDay(date1,date2): One of Arguments is invalid: " + exception.getLocalizedMessage());
        }
    }
}

如果有人需要做聚合函数,那么可能会发现这个例子很有用:

public class MyMax extends Function.Aggregate {

    private long buff = 0;

    public MyMax() {
    }

    @Override
    protected void xStep() throws SQLException {
        long current = value_long(0);
        if (current > buff) {
            buff = current;
        }

    }

    @Override
    protected void xFinal() throws SQLException {
        result(buff);
    }
}

accumulator工作原因是因为在每个查询中都克隆了MyMax instantion,因此在实例化对象时可以提供起始值.

最后一个示例显示如何附加功能:

Function.create(c, IsSameDay.class.getSimpleName(), new IsSameDay());
Function.create(c, MyMax.class.getSimpleName(), new MyMax());

希望有人会发现它有用.

事实证明,使用SQLiteJDBC编写用户定义的函数实际上非常容易.这是一个Groovy示例:

@GrabConfig(systemClassLoader=true)
@Grab('org.xerial:sqlite-jdbc:3.6.16')
import org.sqlite.*
import java.sql.*

db = groovy.sql.Sql.newInstance("jdbc:sqlite::memory:","org.sqlite.JDBC")

// a distance function using the spherical law of cosines
Function.create(db.getConnection(), "distance", new Function() {
    protected void xFunc() throws SQLException {
        def lat1 = value_double(0)
        def lon1 = value_double(1)
        def lat2 = value_double(2)
        def lon2 = value_double(3)

        double theta = lon1 - lon2;
        double dist = (Math.sin(deg2rad(lat1)) * Math.sin(deg2rad(lat2))) + 
            (Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) * Math.cos(deg2rad(theta)))
        dist = Math.acos(dist)
        dist = rad2deg(dist)
        dist = dist * 60 * 1.1515
        dist = dist * 1.609344
        result(dist);
    }

    def deg2rad(deg) {
      deg * Math.PI / 180.0
    }

    def rad2deg(rad) {
      rad * 180.0 / Math.PI
    }
})

db.execute("CREATE TABLE city(name, lat, lon)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('New York City', 40.7143, -74.0060)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('San Francisco', 37.7749, -122.4194)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('Paris', 48.8567, 2.3510)")
db.execute("INSERT INTO city(name, lat, lon) VALUES('Cologne', 50.9407, 6.9599)")

db.eachRow("SELECT a.name as a, b.name as b, distance(a.lat, a.lon, b.lat, b.lon) as d FROM city a, city b WHERE a.name != b.name ORDER BY d;") {
    println "Distance from ${it.a} to ${it.b}: ${it.d}km"
}